The story of Agile is more than just one chapter in the history of software development. It's also an extremely valuable case study in innovation, an elusive and often humbling process that doesn't work in quite the way that we instinctively think it should.
The trajectory of Agile points toward increasing the value of the software delivered. However, it started with no metric of value and almost no notion of what happened outside the development team. Instead, the first phase of Agile, as I describe in the recent publication "Navigating The Future Of Agile And Lean," focused primarily on changing the behavior and world view of developers working together in a team. Therefore, you won't find anything in Scrum or XP that says, "This is how you know your software is better." Instead, these methodologies told you how to work more effectively as a team. Presumably, better results would follow.
Engineering methodologies have been around for a long time. They've not been noticeable for being terribly successful. They are even less noted for being popular. The most frequent criticism of these methodologies is that they are bureaucratic. There's so much stuff to do to follow the methodology that the whole pace of development slows down.